The present invention relates to a transforming circuit applicable to SCPC (Single Channel Per Carrier) multiplex communications, radars, spectrum analyzers and so forth and, more particularly, to a FDM (Frequency Division Multiplex)-TDM (Time Division Multiplex) transforming device for converting a FDM signal into a TDM signal by using a chirp filter.
Prior art FDM-TDM transforming devices of the type using a chirp filter are disclosed in, for example, "A 400ch SCPC SIGNAL DEMODULATOR USING CHIRP TRANSFORM AND CORRELATION DETECTION SCHEME", GLOBECOM TOKYO, November 1987, Session 8, MODULATION CODING FOR SATELLITE COMMUNICATIONS as well as in U.S. Pat. No. 4,759,013 assigned to the applicant of the present invention. In any of the prior art FDM-TDM transforming devices, FDM SCPC signals are swept in frequency by a chirp signal (local frequency-sweeping signal whose frequency varies with the lapse of time) and then fed to a chirp filter. The individual frequency-swept FDM signals applied to the chirp filter are compressed to time locations each being allocated to a different sweeping frequency and then fed out, i.e., the FDM SCPC signals are transformed into TDM signal.
The prior art FDM-TDM transforming devices described above are successful so long as the modulation system imposes no limitations on individual FDM SCPC signals as to frequency band on the frequency axis. However, when it comes to a communication system wherein the frequency band is limited on the frequency axis and intersymbol interference occurs on the time axis, the prior devices allow the integrated value of interference waveform (interference energy) to directly translate into an amount of signal deterioration. Should the FDM frequency interval be not equal to a signal modulation rate multiplied by a natural number, the interference of nearby channels would become critically great.